Pervaporation is a method for removing, concentrating and recovering substances from a liquid by sorbing in a pervaporation membrane the component to be removed, followed by diffusion and evaporation of the component or components to the other side of the membrane followed by condensation. Pervaporation as a method of separation requires the use of non-porous membranes (sometimes referred to as dense membranes), because pervaporation works on a solution-dissolution-evaporation (that is, phase change) mechanism rather than on pore-diffusion as in conventional porous membranes. Pervaporation membranes permit very high separation efficiency for volatile organic compounds, e.g., separation factors of several thousands using pervaporation membranes rather than conventional porous membranes. This high separation efficiency cannot be obtained by porous membranes, through which all molecules diffuse based upon size and shape. The size and shape of molecules to be separated are irrelevant for pervaporation, which means that porous membranes which are suitable for separation by size or molecular shape are not necessarily suitable, and may be particularly unsuitable, for pervaporation processes.
In other words, pervaporation is based upon solution-dissolution-evaporation (i.e., phase change) mechanism, while porous membrane separation is based solely upon the size and shape of molecules involving no phase changes of the components. Porous membranes retain components on the membrane because the molecules of the components are either too large or are the wrong shape to diffuse through the membrane.
Since it is a membrane process, pervaporation is a continuous, non-equilibrium process. Pervaporation is particularly useful for removing and concentrating volatile organic compounds from wastewater. Water has a molecular weight significantly lower than that of chlorinated hydrocarbons, and "pervaporates" two to three orders of magnitude more slowly than chlorinated hydrocarbons through conventional hydrophobic pervaporation membranes. This could not possibly happen in ultrafiltration or microfiltration, as these processes are solely dependent upon discrimination by molecular size and shape.
The major contaminants in industrial wastewater and ground water are volatile organic compounds, particularly chlorinated and aromatic hydrocarbons. Conventional separation technologies such as distillation and liquid--liquid extraction are not applicable because they are prohibitively expensive at the low concentrations of volatile organic compounds generally encountered. Currently, carbon adsorption and air stripping are widely used to remove volatile organic compounds. Neither of these methods can recover volatile organic compounds; the volatile organic compounds are merely transferred from one medium to the next for eventual destruction.
Problems with carbon adsorption are:
(i) Gradual loss of adsorbent capacity with time; PA1 (ii) Spent beds become hazardous wastes; PA1 (iii) There is a possibility of explosion and fire; PA1 (iv) Regeneration of the bed is energy intensive; and PA1 (v) The volatile organic compounds must be further handled upon regeneration. PA1 Two adsorbent beds are required, one for adsorption and one for regeneration, as when one bed is on an adsorption cycle, the other is on a desorption cycle and vice versa; PA1 Steam stripping is routinely employed for regeneration of activated carbon. The energy costs for generating steam are high, and when this steam condenses, it gives rise to a waste water problem; PA1 Activated carbon beds cannot handle large fluctuations in concentrations of volatile organic compounds because of slow kinetics; PA1 The flammability of activated carbon limits the practical adsorption temperature of volatile organic compounds to 120.degree.. Hence, volatile organic compounds with boiling points over 150.degree. cannot be effectively desorbed.
Air stripping merely transfers the volatile organic compounds from aqueous wastes to air in which the volatile organic compounds are diluted and require incineration or other expensive thermal methods for oxidative destruction of the volatile organic compounds.
Pervaporation is essentially a recovery process all in one unit. In most instances, pervaporation is deemed to be cheaper, especially where the recovered volatile organic compounds can be reused, in which case it is also more environmentally friendly.
As noted above, volatile organic compounds can be recovered from water or oil by sorption onto hydrophobic adsorbents such as activated carbon [1-2]. Activated carbon has been used to remove chlorinated hydrocarbons from spilled industrial solvents and gasoline constituents from leaking underground storage tanks, to polish effluents from biological treatments processes, as well as to treat air emissions from air-stripping of groundwater and soil vacuum extraction [3]. These processes operate on adsorption-regeneration cycles.
It has been demonstrated that carbonaceous adsorbents made by polymer pyrolysis can efficiently remove chlorinated hydrocarbons present in small amounts in water. However, these processes use discrete solid phase adsorbent pellets, and operate semi-continuously on adsorption-regeneration cycles. The need for frequent regenerations makes carbon adsorption economically unattractive for high concentrations of volatile organic compounds.
Additionally, activated carbon adsorption suffers from the following disadvantages for control of volatile organic compounds:
The operating life of activated carbon is also limited, since it may lose up to 10% of its activity each time it is regenerated.
In pervaporation, sorption and diffusion determine the separation. In order to combine the advantages of high sorption capacity of microporous hydrophobic adsorbents with the continuous operation of membrane separation processes, a new type of membrane [5] was developed. These heterogeneous membranes consisted of a polymeric phase and adsorbent particles uniformly dispersed into the polymeric phase. For instance, the performance of a silicone rubber membrane for separating alcohol from aqueous solution was enhanced by adding to the membrane an alcohol-selective molecular sieve such as hydrophobic zeolite [5]. Both selectivity and flux were improved according to the results obtained by Hennepe et al. [5]. Hydrophilic zeolite was used to facilitate water transport and to increase the selectivity of water over ethanol in separating ethanol-water azeotrope [6]. Higher oxygen permeability and O.sub.2 /N.sub.2 selectivities were obtained for silicalite (a form of zeolite) filled membranes than for those without silicalite [7]. Duval et al. [8] studied the effect of adding carbon molecular sieves and various kinds of zeolites on the gas separation properties of polymeric membranes. They found that zeolites such as silicalite-1, 13X and KY improved the separation of CO.sub.2 /CH.sub.4 mixtures by rubber polymers. On the other hand, zeolite 5A led to a decrease in permeability and unchanged selectivity. Carbon molecular sieves did not improve the separation performance, or only to a very small extent.
Most of the above-described studies were limited to molecular sieve materials such as zeolites, and to the separation of ethanol-water mixtures by pervaporation or gas separation. However, in these studies the ethanol was present in relatively high concentrations. The "absolute ethanol" obtained has about 0.1% water, which is higher than the concentration of volatile organic compounds in water to be treated. Moreover, pervaporation has heretofore proved to be impractical for use in treating wastewater because most of the reported studies were directed to elucidating the pervaporation operation, and were limited to one-component systems, such as toluene in water. Practical problems, however, invariably involve multi-component systems containing sometimes both polar and non-polar volatile organic compounds.
Strategies for recovery of these multi-component systems can be complex, especially if the volatile organic compounds are to be separated into the pure components for reuse.
Adsorbent-filled membranes have been used for gas enrichment separation, cf. Kulprathipanja et al., U.S. Pat. Nos. 5,127,925 and 4,740,219. Kulprathipanja et al. use adsorbent-filled membranes for separation of oxygen from nitrogen, whereby diffusion appears to play an important role. The gases separated are non-polar atmospheric gases. The adsorbents Kulprathipanja et al. use are zeolite, a crystalline aluminosilicate, activated carbon, inorganic oxides, and ion exchange resin, on a porous polymer which may be polysilicone or cellulose acetate. Moreover, zeolites are essentially cages where the molecule of interest is of a similar size to the cage (pore); the molecule of interest is trapped and later expelled. This is, in essence, a shape-selective adsorbent. Specific gases can be somewhat concentrated by this method, e.g., oxygen in air from 21% to 35%, but not much more. The distinct difference between this process and the pervaporation process is that, unlike pervaporation, which involves volatile compounds, gas separation does not involve a phase change.
Schofield et al., U.S. Pat. No. 5,472,613 discloses using adsorbent-filled membranes for adsorption processes. This is strictly a sorption process, and there is no transmembrane flow involved. Schofield et al. limit the processes to batch-wise adsorption in an equilibrium process, using a thick film (0.1-5 mm) and unspecified solid adsorbents.
Goldberg et al., U.S. Pat. No. 3,862,030, described preparation of microporous membranes by adding an inorganic filler such as silica hydrogel or precipitated hydrated silica, or any other carrier or substrate for volatile matter, into a polymeric matrix. The membranes made by Goldberg et al. are microporous membranes used for filtration, i.e., removal of microscopic or ultrafine particles from the medium in which they are suspended. These membranes thus depend upon pore size for separation, as there are networks of micro-voids or pores formed in the resinous matrix.
Persson et al., U.S. Pat. No. 4,970,085, disclose removing volatiles from an aqueous stream such as a juice. However, the Persson et al. process is limited to use of a solid adsorbent; there is no support such as membrane which sorbs the component to be removed. This process is a sorption process, no membrane is involved, and there is no suggestion of removing and/or recovering volatile organic compounds.